1. Field of the Invention
The present invention generally relates to a light-emitting diode and a method for manufacturing the same and, more particularly, to a light-emitting diode with enhanced brightness and a method for manufacturing such a light-emitting diode with a direct band-gap III-V compound semiconductor material on a GaAs substrate. The present invention relates to an improvement to Taiwan Patent Publication No. 41516 “Light-emitting diode with enhanced brightness and method for fabricating the same” filed by the inventors of present invention.
2. Description of the Prior Art
The light-emitting diode (to be abbreviated as LED hereinafter) has received considerable attention for its advantages such as long lifetime, small size, low heat generation, low power consumption, high response speed, monochromic lighting ability, etc. and has been widely used in applications such as computer peripherals, clock displays, display panels, and many other 3C (computer, communication and consumer) products since the 1960s. Particularly, the development in high brightness LEDs has successfully facilitated the LED applications to extend from indoors to outdoors.
The LED, especially in the range of visible wavelength, has an epitaxial structure of different band-gap types and light-emitting wavelengths according to substrate materials. When a direct band-gap III-V compound semiconductor material such as GaAs is used as a substrate of an LED, the GaAs substrate absorbs a considerable amount of incident light and adversely affects the brightness of the LED. On the other hand, when an indirect band-gap III-V compound semiconductor material such as GaP is used as a substrate of an LED, the LED exhibits enhanced brightness because the GaP substrate is translucent and thus the amount of incident light absorbed by the GaP substrate is comparatively reduced. However, using such an indirect band-gap material complicates the manufacturing process because it suffers from poor light-emitting efficiency and needs to be doped with some impurities to improve the efficiency through lattice vibration or dispersion centers. As a result, the manufacturing cost is increased when the indirect band-gap material is used for the LED.
Accordingly, the inventors of the present invention disclosed a substrate for LED's employing a direct band-gap epitaxial material. The substrate reflects the light emitted from the LED instead of absorbing it such that the light-emitting efficiency is increased, as discussed in Taiwan Patent Publication No. 41516 “Light-emitting diode with enhanced brightness and method for fabricating the same.”
The above prior art is described as shown in FIG. 1, which comprises: an epitaxial LED layer 34 including a P-N junction; a bonded reflection layer 40 under the epitaxial LED layer 34; and a conductive substrate 42 under the bonded reflection layer 40. The light emitted from the P-N junction (as indicated by the dashed line with an arrow a4) is reflected by the bonded reflection layer 40 and travels through the LED layer 34. A back electrode 46 is formed at a bottom surface of the conductive substrate 42. A GaAs substrate 32 and a front electrode 48 are formed in sequence on the epitaxial LED layer 34. The GaAs substrate 32 and a front electrode 48 are designed smaller than the epitaxial LED layer 34 in size such that the amount of the front light (L4) emitting out of the LED can be increased. To simplify the manufacturing process, it is possible that the front electrode 48 and the epitaxial LED layer 34 are designed to have the same area in size such that only the reflected light a4 and most of the lateral light r4 contribute to the emitted light. Under such a configuration, if part of the light emitted from the P-N junction of the epitaxial LED layer 34 travels towards the bonded reflection layer 40 (as indicated by the dashed line with an arrow a4), the light will be reflected by the bonded reflection layer 40 and travel through the LED layer 34, enhancing the LED brightness.
Even though, in the prior art filed by the present inventors, LED's with enhanced brightness can be achieved, there still exist some problems left unsolved as follow:                1. Even though the conductive substrate 42 plays the role that replaces GaAs, the opaque bonded reflection layer 40 limits the LED brightness because part of the reflected light may be absorbed inside the LED.        2. The electric current tends to flow through the shortest and the most conductive path; therefore, most of the light is generated at the center of the P-N junction such that the optical path for the light source is increased and the LED brightness is adversely affected. Moreover, the P-N junction of the LED is not uniformly used.        3. The conductive substrate limits the chosen materials, which may be unfavorable for cost down.        
Therefore, there is need in providing a light-emitting diode and a method for manufacturing such a light-emitting diode so as to improve the light intensity and reduce the manufacturing cost.